1. Field of the Invention
The present invention relates to a control device and a control method for an internal combustion engine, relating to a homogenous-charge compression ignition internal combustion engine which compresses an air/fuel mixture formed in a combustion chamber to cause self-ignited combustion.
2. Description of the Related Art
In a homogenous-charge compression ignition internal combustion engine, an air/fuel mixture formed by mixing a fuel and air in advance is compressed by a piston. As a result, a temperature of the air/fuel mixture reaches a self-ignition temperature to start combustion in a plurality of portions of the combustion chamber in synchronization.
In a common spark ignition internal combustion engine, it is difficult to increase the temperature of the air/fuel mixture to the self-ignition temperature by using an adiabatic compression action. On the other hand, in the homogenous-charge compression ignition internal combustion engine, a compression ratio is set larger than that of the spark ignition internal combustion engine so that an increase in temperature by the adiabatic compression is enhanced. At the same time, means such as an intake-air heater for increasing the temperature of the air/fuel mixture is used. In this manner, the homogenous-charge compression ignition combustion is realized.
With the homogenous-charge compression ignition combustion in which the temperature of the air/fuel mixture reaches the self-ignition temperature to start the combustion in the plurality of portions of the combustion chamber in synchronization, combustion with an air/fuel mixture diluted with a large amount of exhaust gas recirculation (EGR) gas or a super-lean air/fuel mixture, which can suppress an increase in combustion temperature, can be realized. The combustion with the air/fuel mixture diluted with the large amount of EGR gas or the super-lean air/fuel mixture is difficult to realize with conventional flame propagation combustion with spark ignition.
Therefore, in the homogenous-charge compression ignition combustion, the generation of NOx can be suppressed as compared with the conventional flame propagation combustion with spark ignition. Further, a high fuel efficiency improvement effect is provided by the increased compression ratio and the lean combustion. Therefore, the homogenous-charge compression ignition combustion is desired to be realized over a wide operation region.
Moreover, the following control device for an internal combustion engine is also known. In order that the air/fuel mixture temperature may reach the self-ignition temperature by using an increase in temperature caused by heat energy of a burnt gas left in the combustion chamber due to advanced timing of closing an exhaust valve (hereinafter referred to as “internal EGR gas”), an increase in temperature caused by combusting the air/fuel mixture in the vicinity of an ignition device with spark ignition (hereinafter referred to as “ignition assist”), and an increase in temperature caused by a combustion pressure, an internal EGR gas amount and ignition timing are controlled so that a combustion-state parameter converges to a target value at which good combustion is obtained (see Japanese Patent Application Laid-open No. 2011-252471, for example).
However, the related art has the following problem.
In the homogenous-charge compression ignition internal combustion engine, in a high-load operation region in which the fuel amount is large and the combustion temperature is high, the air/fuel mixture temperature increases with the increase in temperature of the internal EGR gas (burnt gas remaining in the combustion chamber) due to the increased combustion temperature. As a result, pre-ignition corresponding to an advanced self-ignition timing or rapid combustion corresponding to a high combustion speed occurs to generate combustion noise.
In order to lower the air/fuel mixture temperature, the internal EGR gas amount is set smaller as the load becomes higher regardless of an engine rpm, as shown in FIG. 13 which shows the setting of the internal EGR gas amount with respect to the engine rpm and the load.
In this case, however, the effect of the EGR gas for lowering the speed of the combustion is also reduced by the reduction in the internal EGR gas amount. Therefore, the combustion noise generated due to the pre-ignition or the rapid combustion cannot be suppressed in the high-load operation region.
Moreover, in the above-mentioned case, even if the increase in the air/fuel mixture temperature caused by the increase in temperature is to be suppressed by retarding the ignition timing, the internal EGR gas temperature, which is higher, is dominant over the air/fuel mixture temperature. As a result, the effect for suppressing the combustion noise cannot be obtained.
As a result, as shown in FIG. 14 which shows the effect of the load and the internal EGR gas amount on a combustion state, in a low-load state in which the combustion temperature is low and the internal EGR gas amount can be increased, the effect of the EGR gas for lowering the speed of the combustion can be obtained. Thus, the region of the internal EGR gas amount in which good combustion is obtained is enlarged.
On the other hand, in a high-load state in which the combustion temperature is high and hence the internal EGR gas amount is inevitably required to be set small, the effect of lowering the speed of the combustion cannot be obtained. Therefore, the region in which the combustion noise may occur becomes larger. Moreover, if the internal EGR gas amount is reduced to suppress the combustion noise, the air/fuel mixture temperature is lowered, and hence the combustion state undesirably enters a misfire region. As described above, in the high-load state, the region of the internal EGR gas amount in which good combustion is obtained is narrower, which limits the operation region.
Further, in the control device for the internal combustion engine described in Japanese Patent Application Laid-open No. 2011-252471, the internal EGR gas amount and the ignition timing are controlled so that a crank angle at which a maximum in-cylinder pressure used as a combustion-state parameter is obtained becomes equal to a target value at which the good combustion is obtained.
For example, in the operation region in which the combustion noise is generated by the pre-ignition or the rapid combustion, the air/fuel mixture temperature is high, and therefore a maximum in-cylinder pressure angle is advanced from the target value. Thus, the air/fuel mixture temperature is lowered by reducing the internal EGR gas amount or retarding the ignition timing. In this manner, the maximum in-cylinder pressure angle is controlled to the target value to obtain the good combustion.
However, the above-mentioned control means the following. Specifically, the combustion state in the combustion-noise region or the misfire region shown in FIG. 14 is merely controlled to be stabilized to be a target specific combustion state in the operation region in which the good combustion is obtained under the same load. Therefore, the operation of the homogenous-charge compression ignition combustion cannot be performed in the high-load state in which a good combustion region is not present.
Further, the stabilization of the combustion at the target value which is a specific condition described in Japanese Patent Application Laid-open No. 2011-252471 corresponds to the stabilization of the combustion at a high temperature at which control margins of the internal EGR gas amount and the ignition timing become smaller, which makes it difficult to enlarge the operation region in which the homogenous-charge compression ignition combustion can be performed.